An LCD device provided with an LCD panel having a wide view is known.
Such an LCD panel includes a liquid crystal mixture which responds to an electric field and changes an orientation direction thereof.
A light which passes through the liquid crystal mixture is deflected according to the orientation direction of the liquid crystal mixture, and a light having luminance according to level of the deflection is emitted from the LCD panel.
Accordingly, the electrodes for generating the electric field are provided in the LCD panel.
An image is displayed by controlling electric potential of the electrodes.
As such LCD panel, a panel using an IPS (In-Plane Switching) type, a VA (Vertical Alignment) type and an FFS (Fringe-Field Switching) type are well known.
Each of the types is characterized by an orientation proceeding of the liquid crystal mixture at the time of displaying an image, structure of a plurality of electrodes, a method of applying a voltage to an electrode, a type of dielectric anisotropy of the liquid crystal mixture, etc.
As for a liquid crystal mixture in which dielectric constant of a long axial direction (parallel direction) thereof is different from dielectric constant of a short axial direction (antiparallel direction) thereof, it is described that the liquid crystal mixture in which the dielectric constant of the long axial direction is lager than the other includes positive dielectric anisotropy, and it is described that the liquid crystal mixture in which the dielectric constant of the short axial direction is larger than the other includes negative dielectric anisotropy.
In the LCD panel of the IPS type, two electrodes to generate an electric field are arranged in parallel at a predetermined interval on one substrate.
An initial orientation direction of the liquid crystal mixture is set in a direction parallel to a face of the substrate. The initial orientation direction of the liquid crystal mixture means an orientation direction which is oriented when an electric field is not applied to the liquid crystal mixture.
When electric potential is given between the two electrodes, an electric field substantially parallel to the face of the substrate generates and the liquid crystal mixture changes the orientation direction thereof according to the electric field.
Accordingly, when the electric field is controlled, the orientation direction of the liquid crystal mixture changes mainly in the face parallel to the substrate.
As such liquid crystal mixture, although a liquid crystal material with positive dielectric anisotropy or negative dielectric anisotropy can be used, the liquid crystal material with the positive dielectric anisotropy is usually used.
On the other hand, the LCD panel of the VA type includes a structure in which the liquid crystal mixture is arranged between substrates facing each other on which electrodes are arranged respectively.
In the LCD panel of the VA type, an electric field is generated in a direction mainly vertical to a substrate face.
The initial orientation direction of the liquid crystal mixture is set in the direction vertical to the substrate face.
When electric potential is applied between the two electrodes, an electric field mainly vertical to the substrate face generates and the liquid crystal mixture changes an orientation direction by responding to the electric field.
Accordingly, by controlling the electric field, the orientation direction of the liquid crystal mixture changes mainly in the face vertical to the substrate.
As the liquid crystal mixture for such LCD panels, liquid crystal materials with negative dielectric anisotropy are usually used.
A contrast deteriorates remarkably when the liquid crystal mixture with positive dielectric anisotropy is used in the LCD panel of the VA type compared with the LCD panel using the liquid crystal mixture with the negative dielectric anisotropy.
When the liquid crystal mixture with positive dielectric anisotropy is used, the initial orientation direction of the liquid crystal mixture is oriented in the direction parallel to the substrate face.
In the LCD panel of the FFS type, the orientation direction of the liquid crystal mixture is controlled according to change of the direction of an electric field generated between electrodes. The following two are known as a configuration of the LCD panel of the FFS type.
One configuration is proposed in Japanese Patent Application Laid-Open No. 1999-316383, Japanese Patent Application Laid-Open No. 2000-10110, Japanese Patent Application Laid-Open No. 2000-89255, Japanese Patent Application Laid-Open No. 2001-56474, Japanese Patent Application Laid-Open No. 2001-59976 and “High Performance mobile application with the High aperture ratio (HFFS) Technology”, Dong hun Lim et al., Asia Display/IDW'13, AMDp=32L, p. 807-80, for example.
In the configuration concerning the proposals, two electrodes are formed close on one substrate. An electric field generates mainly in the direction parallel to the substrate face.
In this case, responding to the electric field which generates at the edges of the electrodes, the orientation direction of the liquid crystal mixture changes mainly in the face parallel to the substrate.
As the liquid crystal mixture of such LCD panel, although the liquid crystal material with positive dielectric anisotropy or negative dielectric anisotropy is used, the liquid crystal material with positive dielectric anisotropy is usually used.
In other configuration, the electrodes are provided on each of two facing substrates, and an electric field mainly vertical to the substrate is generated.
In this case, the orientation direction of the liquid crystal mixture changes mainly in the face vertical to the substrate.
As the liquid crystal mixture, the liquid crystal material with the negative dielectric anisotropy is used like the LCD panel of the VA type.
In the LCD panel of a configuration in which the orientation direction of the liquid crystal mixture changes mainly in the face parallel to the substrate, display irregularity due to a vertical electric field in a direction vertical to the substrate face occurs.
Japanese Patent No. 3481509, Japanese Patent Application Laid-Open No. 2002-31812 and Japanese Patent Application Laid-Open No. 2003-322869 disclose a configuration in which electrodes are arranged on facing substrates and the liquid crystal mixture with the negative dielectric anisotropy is utilized.
FIGS. 21 and 22 are partial cross sections of such LCD panel.
In the LCD panel shown in FIGS. 21 and 22, a first substrate 3100 provided with a first common electrode 3101 and a second substrate 3200 provided with pixel electrodes 3201 are arranged, so that a liquid crystal layer 3300 composed of a liquid crystal mixture of negative dielectric anisotropy is sandwiched therebetween.
The first substrate 3100 includes a polarizer 3102 on a light emitting face, and the second substrate 3200 includes a polarizer 3202 on a light incidence face.
Further, the second substrate 3200 in FIG. 21 includes a second common electrode 3204 and an insulating film 3203, and the pixel electrode 3201 are formed on the insulating film 3203.
On the other hand, the second substrate 3200 in FIG. 22 includes the insulating film 3203, and the pixel electrodes 3201 are formed on the insulating film 3203.
The liquid crystal mixture with the negative dielectric anisotropy has a narrow selection range of physical values, for example, refractive index anisotropy and dielectric anisotropy, compared with the liquid crystal mixture with the positive dielectric anisotropy.
Accordingly, in order to improve display characteristics in the above-mentioned configuration, development of a liquid crystal mixture having a wide selection range of physical values, for example, refractive index anisotropy and dielectric anisotropy, is required.
For example, in order to improve display characteristics, a response time of display characteristics is shortened by using a liquid crystal mixture with low rotation viscosity.
However, since the current liquid crystal mixture with the negative dielectric anisotropy has high rotation viscosity compared with the liquid crystal mixture with the positive dielectric anisotropy, it is difficult to shorten the response time.
In order to improve display characteristics, a method of driving by low voltage is known.
An effective method of driving by low voltage is to use a liquid crystal mixture with large dielectric anisotropy.
However, since the dielectric anisotropy of the liquid crystal mixture with the negative dielectric anisotropy is generally small compared with that of the liquid crystal mixture with the positive dielectric anisotropy, it is difficult to realize the method of driving by low voltage.
Then, Japanese Patent Application Laid-Open No. 2002-365657 discloses an LCD panel in which the liquid crystal mixture with the positive dielectric anisotropy is sandwiched with facing substrates each having an electrode, as shown in FIGS. 23A, 23B.
FIGS. 23A, 23B are a partial cross section of such LCD panel.
The LCD panel includes a first substrate 4100 having a first common electrode 4101, the second substrate 4200 having a pixel electrode 4201, and the liquid crystal layer 4300 composed of a liquid crystal mixture with positive dielectric anisotropy sandwiched with the first substrates 4100 and the second substrate 4200.
The LCD panel further includes a second common electrode 4202 arranged upper the pixel electrode 4201 (the first substrate 4100 side), and the alignment films 4102 and 4203 provided on a side of the liquid crystal layer 4300 of the first substrate 4100 and the second substrate 4200. The alignment film 4203 is formed on the second common electrode 4202.
When electric potential V is applied to the pixel electrode 4201 and the second common electrode 4202, and electric potential Vo is applied to the first common electrode 4101, an electric field Ev is generated in the liquid crystal layer 4300. An liquid crystal molecule M responds to the electric field Ev and is oriented in a direction vertical to the substrate face (refer to FIG. 23A).
When the electric potential V is applied to the second common electrode 4202 and the electric potential Vo is applied to the first common electrode 4101 and the pixel electrode 4201, an electric field EL is generated between the pixel electrode 4201 and the second common electrode 4202.
Then, since an electric field almost not being generated in the liquid crystal layer 4300, the liquid crystal molecule M is oriented in parallel with the substrate face (refer to FIG. 23B).
However, in the above-mentioned constitution, a vertical electric field vertical to the substrate face changes an orientation direction of the liquid crystal mixture mainly in a face vertical to the substrate.
Accordingly, when the image is displayed by changing the orientation direction of the liquid crystal mixture in the face parallel to the substrate, even if above-mentioned constitution is applied, the orientation direction of the liquid crystal mixture does not change greatly in the face parallel to the substrate.
Therefore, improvement of the display characteristics is difficult.
Because the orientation direction of the liquid crystal mixture changes greatly in the face vertical to the substrate by the vertical electric field, it becomes difficult to obtain good display quality.
On the other hand, Japanese Patent Application Laid-Open No. 2006-39369 proposes a semi-transmissive LCD panel in which a transmissive display area which displays an image according to a transmitted light, and a reflective display area which displays an image by a reflected light are arranged within one pixel.
FIG. 24 is a cross sectional view of one pixel in the LCD panel.
The LCD panel includes a first substrate 5100, a second substrate 5200 and a liquid crystal layer 5300 sandwiched between the first substrates 5100 and second substrates 5200.
The ½ wavelength plates 5102 and 5103 and a first common electrode 5104 are provided in a reflective display area R1 of the first substrate 5100.
A first insulating film 5201 is provided on the second substrate 5200.
A second insulating film 5202 and a second pixel electrode 5203 are provided on the first insulating film 5201 of the reflective display area R1.
A first pixel electrode 5204 and a second common electrode 5205 are provided on the first insulating film 5201 of the transmissive display area R2.
Further, a common electrode is not provided on the first substrate 5100 in the transmissive display area R2.
And in the reflective display area R1, an electric field vertical to the substrate face generates between the second pixel electrode 5203 and the first common electrode 5104.
In the transmissive display area R2, an electric field parallel to the substrate face generates mainly between the first pixel electrode 5204 and the second common electrode 5205.
In a reflective display area R1, since an external light enters from a side of the first substrate 5100 is reflected by the second pixel electrode 5203 and an image is displayed, a backlight is unnecessary.
On the other hand, in a place without an external light, by turning on a backlight, a light from the backlight enters from a side of the second substrate 5200 side, and is emitted after being deflected in the transmissive display area R2. Therefore, the image display is possible also in the place without the external light.